1. Field of the Invention
This invention relates to a sun sensor for sensing the attitude of the artificial satellite, the inclination angle of the solar paddle, and so forth.
2. Description of the Related Art
Generally, an artificial satellite measures the incident angle of the sunlight by means of a sun sensor, and on the basis of such measurement, corrects its own attitude and inclination angle of the solar paddle. The following are known types of sun sensors which are currently installed in artificial satellites and which can measure the incident angle of the sunlight with high precision.
(1) An analog type sun sensor which uses a periodic pattern reticle (mask plate with a grid pattern).
(2) A digital type sun sensor made up of a combination of a CCD linear array sensor and a slit.
As is shown in FIG. 1, the type (1) sun sensor has head 21 which has first to fourth light receiving sections A to D, each of which is formed of first and second reticles 22 and 23 and a photoelectric converting element (for example, a solar cell) 24. Each of reticles 22 and 23 has a periodic pattern of light transmission sections and shading sections which are provided in the bank-like form with a predetermined width and arranged in the same direction. The patterns of first reticle 22 in first to fourth light receiving sections A to D are displaced by 0.degree., 90.degree., 180.degree. and 270.degree., respectively from those of second reticle 23 in the corresponding light receiving sections. Only the light which has passed through the light transmission sections of first and second reticles 22 and 23 of each of light receiving sections A to D can reach photoelectric converting element 24.
More precisely, sunlight S incident on head 21 at incident angle .theta. is partly interrupted or shaded by the shading sections of first reticle 22 and then reaches second reticle 23, the resulting light intensity distribution being expressed by function f(.theta.). In addition, sunlight S is also partly interrupted by the shading sections of second reticle 23 before reaching photoelectric converting element 24. Thus, current flowing in photoelectric converting element 24 can be expressed by function g(.theta.) which is a function of incident angle .theta. of sunlight S.
When the distance between first and second reticles 22 and 23 and the widths of the transmission sections and shading sections of respective reticles 22 and 23 are set to proper values, function f(.theta.) representing the light intensity distribution can be approximately expressed as follows by taking the diffusion and diffraction effect of the sunlight into consideration: EQU f(.theta.)=.alpha.+.beta. sin (.beta..theta.)
(.alpha., .beta. and .gamma. are constants)
Assume that current g(.theta.), generated when the sunlight expressed by function f(.theta.) is partly interrupted by reticle 23 and then received by photoelectric converting element 24, can be approximately expressed as follows: EQU g(.theta.)=.alpha.'+.beta.' sin (Y'.theta.)
More precisely, since the patterns of first reticle 22 in first to fourth light receiving sections A to D are displaced by 0.degree., 90.degree., 180.degree.and 270.degree., respectively from those of second reticle 23 in the corresponding light receiving sections, currents g.sub.A (.theta.) to g.sub.D (.theta.) generated in respective light receiving sections A to D of photoelectric converting element 24 are thus expressed as follows: EQU g.sub.A (.theta.)=.alpha.'+.beta.' sin (.gamma.'.theta.) EQU g.sub.B (.theta.)=.alpha.'+.beta.' cos (.gamma.'.theta.) EQU g.sub.C (.theta.)=.alpha.'-.beta.' sin (.gamma.'.theta.) EQU g.sub.D (.theta.)=.alpha.'-.beta.' cos (.gamma.'.theta.)
Current outputs g.sub.A (.theta.) to g.sub.D (.theta.) are supplied to an operation circuit section shown in FIG. 2, from where g.sub.A (.theta.) and g.sub.C (.theta.) are supplied to subtracter 31a which in turn supplies the following output: ##EQU1## Outputs g.sub.B (.theta.) and g.sub.D (.theta.), on the other hand, are supplied to subtracter 31a which in turn supplies the following output: ##EQU2##
Reference signal generator 32 generates sine wave signal sinA and cosine wave signal cosA, used as reference signals, and A is gradually increased. Then, output 2.eta.' sin (.gamma.'.theta.) from subtracter 31a and cosine wave signal cosA are multiplied in multiplier 33a and output 2.beta.' cos (.theta.'.theta.) from subtracter 31b and sine wave signal sinA are multiplied in multiplier 33b. The resultant outputs from the multipliers are supplied to subtracter 34 which in turn supplies output 2.beta.' sin (A-.gamma.'.theta.) to zero-crossing detector 35. Zero-crossing detector 35 generates stop signal S.sub.TP each time the input signal crosses the zero level. Reference signal generator 32 stops changing A in response to stop signal S.sub.TP, and produces A (=.theta.) obtained at this time as an incident angle. Thus, incident angle .theta. can be derived.
However, with the construction as described above, the following problem will occur. The detection precision of incident angle .theta. is determined according to the degree to which function f(.theta.) representing the distribution of the light intensity obtained at the reticle can be approximated to the sine wave. In practice, however, since the sunlight has been diffused and is subjected to diffraction effect at the time of passing through the reticle, it becomes difficult to sufficiently approximate it to the sine wave. Thus, it seems impossible to further enhance the detection precision. Further, it is necessary to provide four sets of two reticles and one photoelectric converting element, increasing the size of the head.
On the other hand, in the sun sensor of the type (2), slit 42 and CCD linear array sensor 45 are provided as main constituents as shown in FIGS. 3 to 5. FIG. 3 shows the construction of the head, FIG. 4 shows the positional relation between slit 42 and CCD linear array sensor 45, and FIG. 5 is a cross sectional view of the device shown in FIG. 3 taken along lines A--A. In FIGS. 3 to 5, 41 denotes a spectral prism, 44 a band pass filter for permitting transmission of only the light having a predetermined wave length, 451 a CCD light receiving element (picture element) and 46 a beam attenuation ND filter.
In the sun sensor described above, the position on the light receiving surface of CCD linear array sensor 45 to which sunlight S having passed through slit 42 is projected is detected based on an output of sensor 45 so as to derive the azimuth of the sun (incident angle of the sunlight) .theta.. In this type of sun sensor, the centroid is detected based on a signal generated from sensor 45 as shown in FIG. 6(a). Alternatively, the center of a wave obtained by filtering the output of sensor 45 is detected as shown in FIG. 6(b) to enhance the detection precision of the incident position of the sunlight on the light receiving surface of sensor 45.
However, in this type of sun sensor, an image made by sunlight S having passed through slit 42 is formed to range only several picture elements to several tens of picture elements, and therefore an output signal of sensor 45 is affected by partial irregularity of CCD light receiving elements (for example, variation in dark currents of the light receiving elements and difference in the sensitivity of the picture elements), making the ability of detecting the position irregular. In this case, uniform ability of position detection can be attained by setting distance h between slit 42 and sensor 45 large so as to enlarge the image made by sunlight S and reducing the maximum detection angle for each picture element or light receiving element. However, this increases the size and weight of the sensor and makes it impractical.
As described above, in the prior art sun sensor of the analog type using the periodic pattern reticle, the detection precision is determined by the diffusion and diffraction effect of the sunlight, and it seems impossible to further enhance the detection precision. The detection precision may be lowered by the error caused in the analog operation, and the problem of increasing the size of the head is caused. Further, in the digital type sun sensor having a combination of the CCD linear array sensor and slit, errors may be easily caused by the partial irregularity of the light receiving elements, and it is necessary to increase the size and weight of the sensor head in order to further enhance the detection precision.